1. Field of the Invention
This invention relates to improvements in superconducting magnets, and more particularly, but not by way of limitation to a novel method and structure for compensating for variations in vapor cooled lead resistance of superconducting magnets due to vapor flow.
2. Description of the Prior Art
Superconductivity is the property of certain materials, at temperatures approaching absolute zero, to carry current without power dissipation. Such materials, at temperatures below a certain critical temperature, Te, have no electrical resistivity, and therefore no I.sup.2 R losses. Coils of such material in liquid helium baths, with currents induced by withdrawing a permanent magnet from a position within the helium, have carried the resulting currents for periods of two years without any voltage drop. The factors controlling superconductivity of such material are the interrelation of magnetic field strength, critical current density, and critical temperature Te. The magnetic field strength, applied externally or generated by a current in the superconductor, limits superconductivity to below certain temperatures and current densities.
The large current-carrying capacity of superconductors provides the basis for very compact, super powerful magnets which can be used in numerous applications where strong magnetic fields are required, for example, in MHD generators, lasers, masers, projectile launchers, accelerators, and bubble chambers. The capital and operating costs of a particular installation using such a magnet in place of a conventional electromagnet would be substantially less due to the smaller physical size and the absence of power consumption or heat dissipation requirements for the magnet itself.
It was found that when a superconducting coil was operated in an open Dewar there was a significant loss of liquid helium to the atmosphere. The liquid helium was then sealed within a vacuum tight Dewar with electrical leads from the magnet power supply, at approximately 70.degree. F., penetrating the vacuum seal to make electrical contact or interface with the magnet coil, at around -425.degree. F. The loss of liquid helium was, thus, reduced markedly.
It then became common to place a shunt across the superconducting magnet to compensate for magnet geometry variations among other things. Examples of such shunts and the reasons for which they have been employed are seen in U.S. Pat. Nos. 3,129,359; 3,187,229; 3,187,236; and 3,278,808.
It has also become common for large superconducting magnets or coils to employ vapor cooled electrical leads for the connection of a magnet power supply to a superconducting magnet. Because of frictional power losses and flux jump power losses within the magnet or coil some helium boil off still occurred within the sealed Dewar. The vapor cooled lead concept took advantage of the boil off or gaseous helium flow by using it to cool the power leads, thereby giving rise to the term "vapor cooled leads". Such vapor cooled leads are described for example in "vapor cooled leads", a performance specification (GDC specification number 11-36805, June 19, 1979).
During normal operation of a superconducting magnet, the magnet or coil will dissipate power due to the noted frictional and flux jump power losses. Characteristically, these losses are not constant thus the helium boil off is not constant and the associated cooling of the vapor cooled leads varies. This variation in coolant flow rate then causes the electrical resistance of the vapor cooled leads to vary due to the temperature coefficient for the particular material. Under normal operating conditions, the change of resistance in the vapor cooled leads would cause the current in the superconducting coil to change, if it were not for regulation of current level by the magnet or coil power supply. The power supply automatically compensates for variations in resistance of the vapor cooled leads and maintains the superconducting magnet or coil at the desired constant current level.
Current shunts have been used for many years to provide an economical method of varying the current (ampere-turns) in the magnet or coil. However, the use of a shunt on a large superconducting coil is not practical if the associated power supply must precisely regulate the amount of current in the superconducting coil or magnet. No matter how well constructed they may be, vapor cooled leads still exhibit electrical power, and thus, dissipate power as heat when a current is passed through them.
If a shunt, as seen in the aforementioned prior art, is now placed across the room temperature terminals of the vapor cooled leads, it effectively isolates the vapor cooled lead flow rate resistance change from the power supply. No corrective action is taken by the power supply since the shunt current would increase. With the customary shunt there would be a net decrease in magnet current and the shunt current would increase thereby masking the magnet current drop from the power supply so that it would be unable to correct for the drop in magnet current regardless of how well it was regulated. Thus, what appeared to be an economical way to vary magnet ampere-turns has been the cause of magnet current drift and resultant magnet field variations that might well be disastrous to a physical process dependent on a constant magnetic field.